1. Field of the Invention
The present invention relates to a propylene block copolymer. More particularly, it relates to a propylene block copolymer which is superior in flexibility, transparency and impact resistance at low temperature.
2. Description of the Related Arts
A crystalline propylene polymer (herein-after, referred to as simply as "propylene polymer") has widely been used in the fields of films, sheets, containers, etc. because of excellent heat resistance and rigidity.
In the medical field such as infusion bag, etc. and food packaging field, materials having a good flexibility, transparency, impact resistance and heat resistance have recently been required. A propylene homopolymer is superior in heat resistance, but is inferior in flexibility, transparency and impact resistance at low temperature. A random copolymer of propylene and .alpha.-olefin is superior in transparency, but is insufficient in impact resistance at low temperature and, therefore, use thereof is limited.
Some trials of improving the impact resistance using a propylene block copolymer have been made.
JP-A-56-084712 discloses a polypropylene block copolymer composed of a propylene ethylene copolymer block as a moiety A and a propylene ethylene copolymer block as a moiety B, characterized in that a melt flow index suitable for an air-cooling inflation method is from 0.01 to 0.3 g/10 minutes, an ethylene content of the moiety A is less than 20% by weight and an ethylene content of the moiety B is not less than 20% by weight. Specifically, a polymer polymerized by a solvent method is disclosed but is not sufficient in transparency. Namely, in Comparative Example of it, there is disclosed a composition having an ethylene content of the moiety A of 4.6% by weight, an ethylene content of the moiety B is 13.9% by weight and a content of the moiety B of 17.7% by weight. Although the transparency is improved by reducing the ethylene content, the impact strength at low temperature is insufficient.
JP-A-06-093061 discloses an ethylene-propylene block copolymer obtained by producing a polymer moiety (component A) which is mainly composed of propylene in the absence of an inert solvent, substantially, in a first step, and then producing an ethylene-propylene copolymer moiety (component B) in a gas phase in a second step, using a Ziegler-Natta catalyst. The ethylene content of the component (B) is from 20 to 50% by weight and the impact resistance at low temperature is good, but the transparency is insufficient.
The present inventors have intensively studied to solve drawbacks of the conventional ethylene-propylene block copolymer. As a result, the problems above can be solved by a specific block copolymer obtained by polymerizing in the absence of the solvent, substantially.